Just take the average, and cut.
It probably will be wrong, but, at least you tried
Which is why the shelves in my house rather redefine the concept of horizontal!!!
As a side note - we have a display thingy in the lounge - made of 3 interlocking circles of wire with3 shelves inside
I made a point of being EXTREMLY careful to try to get it right
spirit level on shelves at all time
I was very pleased with myself when I finished and the level showed it was perfectly level - not even a little bit out
SWMBO took one look at it and said it was wonky again - she is used to it so was not annoyed
BUT I WAS!!!
it was perfect!!
I checked again and it was still just right - I commented that it was the best I have ever done so we looked again
she was looking at one shelf while I was measuring using the other 2
Turns out that the shelves are not level WITH EACH OTHER
so no matter how well you put the thing up - it always looks wonky from one side or the others
Probably the only one I have ever done right - and the manufacturing means that it STILL looks wonky!!!!